“Since the living tribunal was also introduced in the movie, the Multiverse does exist.” Your conclusion does not follow from your premise. The multiverse is a thing in the MCU when the people who make the MCU make it so. Having a character called The Living Tribunal doesn’t do that.
– Paul D. WaiteOct 29 '17 at 12:33

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The MCU is a part of the Multiverse, not the other way around. You even acknowledge that in your question when you cite the MCU's Multiverse designation. It's five 9s, btw, Earth-199999.
– J DoeFeb 13 '18 at 22:38

2 Answers
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Yes, the Multiverse does exist in the MCU and it is stated as such in Doctor Strange as you mention.

The Ancient One: Learning of an infinite multiverse included learning of infinite dangers. And if I told you everything else that you don’t already know, you’d run from here in terror.

Doctor Strange

It is worth noting though that there are other mentions of the Multiverse in Dcotor Strange, however, most of them appear to talk about the other dimensions rather than other universes. And so these mentions of the Multiverse are not actually talking about the Multiverse itself.

The Ancient One: The language of the mystic arts is as old as civilization. The sorcerers of antiquity called the use of this language "spells". But if that word offends your modern sensibilities, you can call it "program". The source code that shapes reality. We harness energy drawn from other dimensions of the multiverse, to cast spells, conjure shields and weapons to make magic.

Doctor Strange

Whilst we're on the note of false positives we should explore some things like the Multiverse but aren't actually it:

Dimensions - as previously mentioned different dimensions, Astral, dark, etc., aren't actually different universes or part of the Multiverse and these are what we see throughout Doctor Strange.

Alternate timelines - A major plot of Avengers: Endgame is the Time Heist which creates different timelines. These alternate timelines can be created and erased by removing something from the "past" or putting it back but these aren't the Multiverse.

Mysterio - In Spider-Man: Far From Home Mysterio tells everyone he is from Earth, just not ours. He even gives the Earth's numbers , 833 for his and 616 for ours. However, he was lying and it was all an elaborate story and so this isn't the Multiverse and the numbers weren't real.

The Nine Realms - These are the Realms, Asgard, Midgard, etc., and aren't the Multiverse.

We have no current further confirmation of a Multiverse as far as I am aware in the MCU. However, it is important to note that the Doctor Strange squeal is to be titled Doctor Strange in the Multiverse of Madness which would indicate that yes the Multiverse does exist in the MCU. However, until the film is out we do not know for sure.

From an out of universe perspective it is of course worth noting that the MCU is part of the "normal" Marvel Multiverse with it being listed as Earth-199999 and the main comics continuity is Earth-616. It's just that currently no other universe has interacted with the MCU yet.

The MCU is one universe that is part of the Multiverse. It is parallel to the universe of Howard the Duck (who was in Guardians of the Galaxy). It is also parallel to the main comics universe, the X-Men movie universe, the X-Men cartoon, universe (and basically all the others in the Marvel wiki).

Parallel universes exist independently of each other. Remember, the Infinity Stones came from some other part of the Multiverse.
They introduced the idea in Doctor Strange so viewers will have at least a clue of what's going on if they reboot the whole franchise, or for example, they get the mutant characters film rights back, the X-Men stories have a lot of internal history, so if they get the rights back, they could either have the beginnings of mutants showing up, causing a lot of changes to backstories like Magneto, who've been doing their very public thing for decades.

Otherwise, they'd have to explain how no one had even heard of the millions of mutant characters, so I'd bet they'd either reset the whole universe so there were mutants all along, or they'd remind their viewers of the multiverse, and have the existing mutants come on over from their current Fox-owned universe, even played by the same actors.

I rather understood that the occurrence of Howard the Duck was a little joke, not meant to be explored further.
– Mr ListerNov 1 '17 at 16:11

Howard the Duck is from Earth, just one from another universe. It wouldn't doesn't make any
– LemonFireNov 1 '17 at 16:13

It wouldn't make sense to just have him be from a seperate planet, it only works if he's from the Earth that's remarkably similar to ours, just the people are ducks, not primates.
– LemonFireNov 1 '17 at 16:15

Sorry, I meant the appearance of Howard the Duck in GotU. Not the whole phenomenon of Howard in general.
– Mr ListerNov 1 '17 at 17:16

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